In ReSharper, to add access modifiers or not is a formatting preference. You can change it at ReSharper | Options -> Code Editing | C# | Formatting Style | Other | Modifiers
. Turn on Use explicit ...
to always add modifiers when making code cleanup or turn them off not to do it.
But ReSharper can't highlight or underline places where wrong formatting style is used (for example, where there is no access modifiers). You should try StyleCop for this.